1. Field of the Invention
This invention relates to a method for producing a tin-zinc alloy film, particularly a method for producing a tin-zinc alloy film for corrosion-resistant purposes.
2. Description of the Prior Art
Recently, an attention has been paid to a tin-zinc alloy film to replace for a corrosion-resistant cadmium plating film. Such a cadmium plating film itself has excellent corrosion-resistance and thus, is employed as a material for an aircraft at present. However, the cadmium plating film is strictly restricted in use because it has harmful element, Cd for environment. In this point of view, the use of the cadmium plating film would be more severely restricted in future. Therefore, the tin-zinc alloy film would play very important roles in future instead of the cadmium plating film.
In the past, the tin-zinc film would be produced from a given water solution by means of alloy-electroplating, and thus, two different metals, tin and zinc, must be electroplated at the same voltage. Therefore, all kinds of things to perform the electroplating at the same voltage have been carried out. Moreover, chemical species to be used have been restricted, and anti-environmental additives have been used.
Furthermore, the above electroplated tin-zinc film from the water solution always has a thermally non-equilibrium phase which is not recognized in its equilibrium diagram. The non-equilibrium phase often shifts to another stable phase due to the wear or the heating in the use of the tin-zinc alloy film, and thus, the properties of the tin-zinc alloy film often change. Therefore, the functions imparted to the tin-zinc alloy film for predetermined purposes may change during the use of the film, so that it may be that the tin-zinc alloy film can not exhibit the predetermined functions in use.
Such a technique is described in Japanese Patent Application Laid-open No. 01-165791 as plating a zinc film and a tin film in their respective predetermined thickness on a given steel plate and then, melting the tin film and diffusing the tin elements into the zinc film through a given thermal treatment, to produce a tin-zinc alloy film. With such a technique, however, the tin elements and the zinc elements are inclined in the tin-zinc alloy film in concentration, so that the tin elements and the zinc elements can not be alloyed perfectly. Therefore, the imperfect tin-zinc alloy film may change in property, and thus, can not exhibit their functions imparted in advance sufficiently.